Black electrophoretic particles and method of manufacture

ABSTRACT

The present invention are dielectric black particles for use in electrophoretic images displays, electrostatic toner or the like, and the corresponding method of manufacturing the same. The present invention black particles are latex particles formed by a polymerization technique, wherein the latex particles are stained to a high degree of blackness with a metal oxide.

This is a Division of application Ser. No. 07/901,755, filed Jun. 22,1992, now U.S. Pat. No. 5,298,833.

FIELD OF THE INVENTION

The present invention relates to dielectric black particles for use inelectrophoretic image displays, electrostatic toner or the like and thecorresponding method of manufacturing the black particles. Moreparticularly, the present invention relates to polymer latexes, preparedby an emulsion polymerization technique, wherein the polymer latexes arereacted with a metal oxide forming black dielectric particles that havephysical characteristics, such as size, density, and surfacefunctionality selectively determined by varying the polymerizationreaction.

Background of the Invention

The electrophoretic effect is a well known and the prior art is repletewith a number of patents and articles which describe the effect. As willbe recognized by a person skilled in the art, the electrophoretic effectoperates on the principle that certain particles, when suspended in anmedium, can be electrically charged and thereby caused to migratethrough the medium to an electrode of opposite charge. Electrophoreticimage displays (EPID) utilize the electrophoretic effect to producedesired images. In prior art EPID colored dielectric particles aresuspended in a fluid medium that is either clear or of an opticallycontrasting color as compared to the dielectric particles. The coloredelectrophoretic particles are then caused to selectively migrate to, andimpinge upon, a transparent screen, thereby displacing the fluid mediumagainst the screen and creating the desired image.

As will be recognized by a person skilled in the art, the selection ofthe electrophoretic particles used in the EPID is very important indetermining the performance of the EPID and the quality of the viewedimage produced. Ideally, electrophoretic particles should all be of auniform size, to help in assuring that each of the electrophoreticparticles will behave similarly. Additionally, it is desirable toutilize electrophoretic particles that have essentially the same densityas the fluid medium in which they are suspended. By usingelectrophoretic particles of essentially the same density as thesuspension medium, the migration of the electrophoretic particlesthrough the medium remains independent of both the orientation of theEPID and the forces of gravity.

To affect the greatest optical contrast between electrophoreticparticles and the suspension medium, it is desirable to have eitherwhite particles suspended in a black medium or black particles suspendedin a backlighted clear medium. In the prior art, it has proven difficultto produce black electrophoretic particles that are dielectric, ofuniform size, and which have a density matching that of a commonsuspension medium. As a result, EPIDs commonly use readily manufacturedlight colored electrophoretic particles suspended in dark mediums. SuchEPIDs are exemplified in U.S. Pat. Nos.: 4,655,897 to DiSanto et al.,4,093,534 to Carter et al., 4,298,448 to Muller et al., and 4,285,801 toChaing. In such prior art where light particles are suspended in a darkmedium, the suspension often appears grayish until the application of anelectric field. With the electric field applied, the light coloredparticles migrate through the grayish suspension producing a light imageon a gray background, thereby resulting an image that is not highlycontrasted.

To produce a more contrasted image, it is desirable to backlightsuspended black particles in a clear medium. However, as has beenmentioned, the development of suitable dielectric black particlesremains a long felt need in the art of electrophoretic image displays.In arts other than EPIDs, black particles are commonly produced fromcarbon. However, carbon blacks are not readily adaptable to EPIDsbecause carbon blacks are conductive and the density of carbon blacks isnot readily matched to a suitable suspension medium. Research effortshave been made in an attempt to solve the density and conductivityproblems of carbon blacks, however, none has succeeded without tradingoff the blackness (i.e. energy absorbency) of the particles created.Such efforts to produce dielectric particles from carbon blacks areexemplified in the following articles: Fowkes et al. "ElectrophoreticDisplay Medium", a research project report for the Department ofChemistry at Lehigh University (Aug. 28, 1989) and Hou et al."Polymer-Encapsulated Particles With Controlled Morphologies," PH.DDissertation, (Lehigh University, 1991).

The present invention does not use carbon black as the source of theelectrophoretic particle. Rather, composite latexes stained with a metaloxide are used to form the dielectric black particles suitable for usein a EPID. More particularly, the preferred embodiment of the presentinvention produces black particles from seeded emulsion polymerizationtechniques, used to produce core/shell latex structures with theresidual double bonds, that are stained black with a metal oxide. Thedensity, blackness, particle size and surface characteristics of thepresent invention black particle are controlled by the polymercomposition, crosslink density, residual double bond density andreaction conditions.

The development of particles from synthesized core/shell latexes hasbeen addressed in numerous technical articles, as exemplified by thefollowing: Wessling et al. "A Study Of Emulsion Polymerization Kineticsby the Method of Continuous Monomer Addition." Journal of MacromolecularScience., A7 (3), pp. 647-676 (1973); Keusch et al. "The Growth ofPolystyrene Latex Particles." Journal of Macromolecular Science., A7 (3)pp. 623-646 (1973); Grancio et al. "The Morphology of theMonomer-Polymer Particle in Styrene Emulsion Polymerization." Journal ofPolymer Science., vol. 8, pp 2617-2629 (1970); Grancio et al. "MolecularWeight Development in Contrast-Rate Styrene Emulsion Polymerization."Journal of Polymer Science., vol. 8, pp. 2733-2745 (1970); and Wiener,H. "Polymerization in the System Vinylidene Chloride-PotassiumLaurate-Potassium Persulfate", Journal of Polymer Science vol. 7, pp.1-20 (1951). Of more direct relation to the present invention process ofproducing black particles are the below referenced articles.

In Daniels et al. "Preparation of Acrylonitrile/Butadiene/StyreneLatexes Using Hydroperoxide Redox Initiators." Journal of AppliedPolymer Science, vol. 41, pp. 2463-2477 (1990), anacrylonitrile/butadiene/styrene composite latex is shown where thepolybutadiene core is uniformly surrounded by apoly(styrene-co-acrylonitrile) shell. In Daniels, batch andsemi-continuous seeded emulsion polymerization techniques are used withvarying core and shell ratios and other reaction parameters.

In Merkel, M. P. "Morphology of Core/Shell Latexes and their MechanicalProperties" PH.D. Dissertation (Lehigh University 1986), seeded emulsionpolymerization is utilized to synthesis polybutadiene-poly(methylmethacrylate) core/shell latexes. The core/shell latexes have variouslevels of crosslink density of the core and various thickness of theshell.

In Sundberg et al. Journal of Dispersion Science Technology vol. 5, pp.433 (1984), synthesized polybutadiene-polystyrene core-shell latexes arestudied in various conditions, such as monomer/polymer ratio, initiatorlevel, degree of conversion and concentration of chain transfer agent,to determine grafting efficiencies of styrene onto polybutadienelatexes.

The use of staining agents on polymers was first used to form contrastshelpful in viewing polymer structures through electron microscopy. Theprior art pertaining to such polymer staining is exemplified in thefollowing articles. In Gaylarde et al. Science, vol 161, pg. 1157 (1968)ruthenium tetroxide was used as a staining agent for polymeric materialsin electron microscopy. In Vitali et al. Polymer, vol 21, pg. 1220(1980) ruthenium tetroxide was used to improve image contrast forpolybutadiene lattices, a terpolymer of acrylnitride, butadiene andstyrene, and an acrylnitrile-styrene-acrylnitrile polymer. In Trent etal. Journal of Polymer Science, volume 19, pg. 315 (1981), rutheniumtetroxide was used in vapor stainingpolystyrene/poly(methylmethacrylate) blends for electron microscopystudies. Finally, in Trent et al. Macromolecules, vol 16, pg. 589 (1983)ruthenium tetroxide was shown having the capability of staining bothsaturated and unsaturated polymer systems that contain ether, arene,alcohol, aromatic, amide, or olefin moieties.

However, none of the above referenced prior art addresses a process ofproducing dielectric particles having a high degree of blackness with acontrolled density and particle sizes so as to be adaptable to anelectrophoretic image display. It is, therefore, an object of thepresent invention to provide an improved electrophoretic particle thathas a high degree of blackness, a controlled particle size, surfacefunctionality and density so as to be readily suspended in the liquidmedium of an electrophoretic image display.

SUMMARY OF THE INVENTION

The present invention relates to black dielectric particles that can beused in an electrophoretic image display and the corresponding method ofproducing the same. To produce the present invention black particles,latex particles are produced using an emulsion polymerization technique.The latex particles produced contain residual double bonds which arethen reacted with a metal oxide to produce a desired degree ofblackness.

In a preferred embodiment the latex particles are formed having a corepolymer surrounded by at least one shell of a differing polymer. Thecore structures are polymers with residual double bonds such aspolybutadiene and polyisoprene and the shell structures are polymerswith different functionalities such as poly(methacrylic acid) andpoly(methyl methacrylate) or copolymers such aspoly(styrene-co-methacrylic acid) and poly(styrene-co-methylmethacrylate). The residual double bonds of the core structures are thenreacted with a metal oxide such as osmium tetroxide to form a complexstructure which efficiently absorbs incident light and provides a highdegree of blackness. The blackness of the core/shell particles isdependent upon the residual double bond density of the core, the shellthickness, the core/shell ratio, particle size and the degree ofstaining. The surface characteristics of the particles are controlled bythe chemical composition of the shell structure. The black particlesalso have controlled mechanical properties which are dependent upon thecrosslink density as well as the composition of the core and shellstructures. Since the core structure and shell structure can be madefrom differing polymers and formed in any ratio, the density of theblack particle can be controlled by selecting the size of the particleto be made, the latex material to be used and the ratio of the corestructure to the shell structure. As such, the present invention blackparticles can be formed with a desired density so as to match thedensity of a desired suspension medium in a electrophoretic imagedisplay.

The sole FIGURE is a cross sectional view of an electrophoretic displaycontaining dielectric black particles produced in accordance with theone preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the present invention black particles can be used in manydifferent applications where particles of high blackness and low densityare desired, such as paint, ink, and electrostatic toner, it isespecially suitable for use in connection with electrophoretic imagedisplays (EPID). Accordingly, the present invention black particles willbe described in connection with a typical EPID.

Referring to FIG. 1, there is shown a cross sectional view of anelectrophoretic image display 10. As will be recognized by a personskilled in the art, an EPID 10 contains a volume of an electrophoreticdispersion 12 encapsulated between a anode structure 14 and a cathodestructure 16. The cathode structure 16 is comprised of glass plate 17 onwhich is deposited a thin layer 18 of indium-tin-oxide (ITO) or a likecompound. The ITO layer 18 is deposited in such a manner so as to besubstantially transparent when viewed through the glass plate 17.Cathode lines 20 are etched onto the ITO layer 18 in a pattern ofparallel lines. The anode has a similar set of parallel ITO lines 26,which are perpendicular to those on the cathode. A conductive mesh 27sits in the fluid between the cathode and anode.

In the present invention, the electrophoretic dispersion 12 is comprisedof black dielectric electrophoretic particles 22 suspended in a clearmedium 24. The electrophoretic particles 22 have a density substantiallyequivalent to that of the fluid medium 24 so as to remain randomlydisperse in the fluid medium 24, unaffected by the orientation of theEPID 10 or the effects of gravity. When an electrical bias is applied tothe cathode lines 20, the anode lines 26 and the mesh in propersequence, the electrophoretic particles 22 migrate to selected positionson the cathode lines 20 creating an image viewable through the glassplate 17. The migration of the black electrophoretic particles 22 to thecathode displaces the clear medium 24 adjacent to the ITO layer 18,thereby blocking the light produced by the light source 25. Theremainder of the particles 22 sit on the mesh and do not block lightfrom source 25. Consequently, black electrophoretic particles 22 can beseen through the glass plate 17, resulting in a black image contrastedagainst a light background.

As has been indicated previously in the Background of the Invention, theproduction of a black image on a light background is highly desirable.However, a major problem associated with any EPID is the creation ofdielectric black particles that have a density that can be readilymatched with common suspension fluids. The present invention blackelectrophoretic particles 22 are formed from composite latexes usingseeded emulsion polymerization techniques. The composite latexes arethen reacted with a metal oxide which stains the residual double bondsof the latexes black. Latexes are dielectric materials, as are metaloxides, consequently the resulting black particles formed from thelatexes are dielectric. Additionally, utilizing varying core and shelllatexes, black particles of low density can be readily obtained. Byvarying the latex materials used, size of the particles produced and theratio between the core and shell polymers and the density of the blackparticles can be selectively adjusted. The resulting black particles canthereby be formed to have a desired density, in the rage of 0.8 to 1.4gm/cm³, allowing the black particles to have the same density as manysuspension media.

To form the present invention dielectric black electrophoretic particles22, core/shell latex structures are prepared by batch andsemi-continuous seeded emulsion polymerization techniques. As will berecognized by a person skilled in the art, the creation of a core/shellcomposite latex by seeded emulsion polymerization includes synthesizinga seed latex by traditional emulsion polymerization, swelling the seedparticles with a second stage monomer and then polymerizing the secondstage monomer during the second stage polymerization to encapsulate theseed particles within a newly formed shell polymer. In the presentinvention, the core structures are manufactured as polymers containingresidual double bonds, such as polybutadiene or polyisoprene. The shellstructures are manufactured as polymers having different functionalitiesfrom the core structure, such as poly(methacrylic acid) and poly(methylmethacrylate) or copolymers such as poly(styrene-co-methacrylic acid)and poly(styrene-co-methyl methacrylate). The residual double bonds ofthe core structures are then stained with a metal oxide to form acomplex structure with a high degree of blackness, wherein the degree ofblackness is dependent upon the residual double bond density of the corestructure, the thickness of the shell structure, the ratio between thecore and shell structures, the overall size of the core/shell structureand the degree of the reaction with the metal oxide.

PROCESS ONE

In an exemplary embodiment of the present invention method for producingthe electrophoretic black particles 22, the core structure, commonlycalled a seed latex is prepared by emulsion polymerization in a closedcontainer. The core structure is a crosslinkedpoly(butadiene-co-styrene) latex formed from the polymerization recipedescribed in Table I below.

                  TABLE I                                                         ______________________________________                                        Materials            Weight (g)                                               ______________________________________                                        styrene              8.0                                                      butadiene            35.0                                                     distilled-deionized water                                                                          100.0                                                    dioctyl sodium sulfosuccinate                                                                      0.07                                                     octyl phenoxy polyethoxy ethanol                                                                   0.24                                                     potassium persulfate 0.4                                                      divinylbenzene       0.4                                                      ______________________________________                                    

Prior to use, the inhibitors of the monomers butadiene and styrene andthe crosslinker divinylbenzene are removed by standard methods ofpassing each of the monomers through a column containing the appropriateinhibitor remover. In this, and subsequent processes, the butadiene isof the type commercially available from Air Products and Chemicals, Inc.The styrene is of the type commercially available from FisherScientific, Inc., and the divinylbenzene is of the type commerciallyproduced by Dow Chemicals, Inc.

The octyl phenoxy polyethoxy ethanol (also known as Triton X-45manufactured by Rohm and Haas, Inc.) and dioctyl sodium sulfosuccinate(also known as Triton X-200 manufactured by Rohm and Haas, Inc.) areused as emulsifiers and are dissolved in the distilled-deionized waterand charged into a closed container. The styrene and divinylbenzene arecombined and mixed with an initiator of potassium persulfatemanufactured by Fisher Scientific, Inc. After the styrene,divinylbenzene, and potassium persulfate are combined, the combinationis charged to the closed container containing the Triton X-200 andTriton X-45 emulsifier solution. The closed container is then purgedwith nitrogen. The butadiene monomer is then condensed and weighted intothe closed container. The butadiene can be considered with any knownmethod but is preferably condensed using isopropanol cooled with liquidnitrogen. The container, containing the composite mixture is the warmedand agitated for a desired reaction time. In one preferred embodimentthe mixture is tumbled at fifteen revolutions per minute for forty eighthours at seventy degrees celsius. At the end of the forty eight hourperiod, the core structures of poly(butadiene-co-styrene) namely, seedparticles, are formed that are monodisperse in size having a diameter ofapproximately 286 nm, as they would appear in a TEM photograph

A shell structure of poly(styrene-co-methacrylic acid) is formed aroundthe poly(butadiene-co-styrene) core structure by batch seeded emulsioncopolymerization. The shell structure for thepoly(styrene-co-methacrylic acid) polymer is formed from thepolymerization recipe described in Table II below.

                  TABLE II                                                        ______________________________________                                        Materials          Weight (g)                                                 ______________________________________                                        poly(butadiene-co-sytrene)                                                                       5.0                                                        seed latex                                                                    styrene            1.5                                                        methacrylic acid   0.2                                                        divinylbenzene     0.1                                                        potassium persulfate                                                                             0.02                                                       distilled-deionized water                                                                        40.0                                                       ______________________________________                                    

To form the poly(styrene-co-methacrylic acid) shell structure around thepoly(butadiene-co-styrene) core structure the poly(butadiene-co-styrene)seed latex are placed in a container with the styrene, divinylbenzeneand a methacrylic acid monomer, such as that manufactured by AldrichChemical, Inc. The container is then purged with nitrogen and thepoly(butadiene-co-styrene) core structures are swelled in the presenceof the other monomers at room temperature. Thepoly(butadiene-co-styrene) core structures are swelled to a desireddegree and are then combined with the potassium persulfate initiator. Inthe preferred embodiment, the combination is then heated to sixtydegrees celsius and tumbled at thirty revolutions per minute for twentyfour hours. As a result of the above process,poly(butadiene-co-styrene)/poly(styrene-co-methacrylic acid) core/shellstructures are produced having a diameter of approximately 480 nm ofwhich approximately 50 nm is a result of the poly(styrene-co-methacrylicacid) shell structure thickness.

Two percent, by weight, aqueous solution of osmium tetroxide is thenadded to the core/shell latex so as to react with, and stain, theresulting residual double bonds. The core/shell latex is tumbled withthe osmium tetroxide solution at room temperature for a desired reactiontime, thereby resulting in a core/shell latex, having a desired degreeof blackness, that can be used as the present invention electrophoreticparticles 22. In regard to the advantages set forth hereafter in processtwo. It should be understood that in place and stead of the osmiumtetroxide, ruthenium tetroxide or other metal oxides may also be used.

The above described method of manufacture, utilizing thepoly(butadiene-co-styrene) core structure produced by the polymerizationrecipe of Table I and the poly(styrene-co-methacrylic acid) shellstructure produced by the polymerization recipe of Table II, produceblack particles of a given blackness, size, hardness and surfacecharacteristics. By varying the polymerization recipes of Tables I andII and by varying other reaction parameters of the method ofmanufacture, the physical characteristic of the black particles producedcan be selectively altered as needed for a given application.

By varying the degree of conversion and the amount of the divinylbenzenecrosslinker present in the creation of the poly(butadiene-co-styrene)core structures, the concentration of the residual double bonds presentin the poly(butadiene-co-styrene) core structure can be altered. Thedegree of conversion during polymerization is dependent upon thereaction time which can be varied from twenty four hours to seventy twohours to obtain fifty percent to ninety nine percent of conversion. Ahigher degree of conversion leads to fewer residual double bonds left inthe core structure. The polymerization recipe set forth in Table I calls0.4 gms of divinylbenzene. However, the amount of divinylbenzene can bevaried from 0.0 gm to 1.2 gms. Consequently, the concentration ofresidual double bonds will vary as a function of the concentration ofdivinylbenzene within the given range. Since the residual double bondspresent in the core/shell structure is what reacts with the metal oxide,by varying the concentration of residual double bonds the blackness ofthe end product electrophoretic particles can be selectively varied asdesired. Additionally, by varying the concentration of residual doublebonds, the hardness of the end product electrophoretic particles canalso be varied within the available range.

The blackness and hardness of the produced particles may also beeffected by altering the butadiene:styrene monomer ratio used in thepoly(butadiene-co-styrene) core structure polymerization recipe of TableI. In Table I, the given butadiene:styrene monomer ratio was 35:8. Inorder to selectively control the hardness and blackness of the endproduct electrophoretic particles, the butadiene:styrene ratio in thepoly(butadiene-co-styrene) core structure can be changed form the TableI value of 35:8 to 39:4, 31:12, 27:16, or 23:20 as desired. In additionto butadiene, other conjugated diene compounds (e.g. isoprene) or anycompound containing more than one double bond (e.g. diacrylate,triacrylate, tetraacrylate, dimethacrylate and trimethacrylatecompounds) can also be used as stained component.

In emulsion polymerization there are many variables such as swell time,reaction time and polymerization recipe that can be altered to affectthe size of the formed core structure and the ratio between the size ofthe core structure and the surrounding shell structure. In the presentinvention, the size of the poly(butadiene-co-styrene) core structuresare preferably controlled by varying the concentration and types ofemulsifiers present in the poly(butadiene-co-styrene) core structurepolymerization recipe. In the polymerization recipe of Table I, 0.07 gmsof Triton X-200 and 0.24 gms of Triton X-45 emulsifiers are used. Itshould be recognized by a person skilled in the art that by varying theconcentrations of the emulsifiers during polymerization, the size of thepoly(butadiene-co-styrene) core structure can be selectively controlled.Preferably in the polymerization recipe of Table I, the Triton X-200emulsifier can be varied from between 0.0 gms and 0.14 gms while theTriton X-45 emulsifier can be varied from 0.16 gms to 0.40 gms. Anionic,cationic and nonionic emulsifiers or the combination for each type ofemulsifier can be used in the emulsion polymerization. Specific examplesof suitable emulsifiers are sodium lauryl sulfate, sodium dodecylsulfate, Dowfax surfactants, lgepal surfactants, Aerosol surfactants,Pluronic surfactants, Cantrez surfactants, Arlacel surfactants, Tetronicsurfactants, poly(vinlyalcohol), poly(ethylene oxide), and the like.

The physical characteristics of the end product electrophoreticparticles can also be varied by varying the process and polymerizationrecipe for the poly(styrene-co-methacrylic add) shell structure thatsurrounds the poly(butadiene-co-styrene) core structure. For example,the surface functionality of the final core/shell structure can bevaried by varying the amount of methacrylic acid monomer present duringpolymerization. By varying the amount of methacrylic acid monomer fromthe 0.2 gms listed in Table II to 1.0 gm, the amount of carboxylic acidon the shell structure is changed thereby which affecting a change inthe surface functionality of the end product electrophoretic particles.Additionally, it should also be understood that the use of a methacrylicacid monomer is exemplary and the surface functionality of the endproduct electrophoretic particles can be changed by substituting othermonomers such as methyl methacrylate, acrylonitrile, vinyl chloride,acrylic acid, sodium styrene sulfonate vinyl acetate, chlorostyrene,dimethylamino-propylmethacrylamide, isocyanatoethyl methacrylate,N-(iso-butoxymethyl) acrylamide, or other similar functional monomers inplace and stead of the methacrylic acid.

The shell thickness of the poly(styrene-co-methacrylic acid) shellstructure, as well as the blackness and mechanical properties of the endproduct electrophoretic particles, can be selectively altered by varyingthe monomer:polymer ratio used in the second stage polymerizationrecipe. In the preferred embodiment of the second stage polymerizationrecipe, shown in Table II, the monomer:polymer ratio can selectivelyadjusted from 80:20 to 20:80 in order to affect the needed shellstructure characteristics.

PROCESS TWO

In an alternative embodiment of the present invention, black particlesare produced by emulsion polymerization which producespoly(styrene-co-methacrylic acid) latex particles. In this embodimentpoly(styrene-co-methacrylic acid) latex particles are prepared utilizingthe polymerization recipe shown below in Table III.

                  TABLE III                                                       ______________________________________                                        Materials         Weight (g)                                                  ______________________________________                                        styrene           40                                                          methacrylic acid  2                                                           distilled-deionized water                                                                       100                                                         potassium persulfate                                                                            0.4                                                         divinylbenzene    0.4                                                         sodium lauryl sulfate                                                                           1.2                                                         ______________________________________                                    

In preparing the poly(styrene-co-methacrylic acid) latex particles, thecontents of Table III were charged into a container, purged withnitrogen and agitated for a desired reaction time to completepolymerization. The resulting poly(styrene-co-methacrylic acid) latexparticles are then either mixed with a two percent by weight aqueoussolution of ruthenium tetroxide or exposed to ruthenium tetroxide vaporto stain the polystyrene component of the polymer particles. Because theinterfacial tension between poly(methacrylic acid) and water is lowerthan that between polystyrene and water the methacrylic acid componentwill migrate to the surface of the particles during polymerization,which provides functional groups for surface charging.

Particles made by this process are spherical and are uniform in size,namely, monodisperse particles, which allow each particle to haveuniform surface charge and charge/mass ratio to create uniformelectrostatic images in electrophoretic image displays.

Poly(styrene-co-methacrylic acid) latex particles with different surfacefunctionalities for surface charging can be formed by changing thefunctional monomer of methacrylic acid to other functional monomers suchas acrylic acid, methacrylate, vinyl acetate, methyl methacrylate,acrylonitrile, sodium styrene sulfonate, dimethylaminopropylmethacrylam,isocyanatoethyl methacrylate, N-(iso-butoxymethyl) acrylamide, or othersimilar functional monomers.

Properties such as density, optical property glass transitiontemperature and mechanical strength of the poly(styrene-co-methaclryicacid) latex particles can be selectively altered by substituting thestained component form styrene to another vinyl monomer such as vinylmethyl ether, vinyl formal, vinyl alcohol, vinyl methyl ketone,ethylene, propylene or combination of the above to make homopolymer, orcopolymer particles

The size of the poly(styrene-co-methacrylic acid) latex particles canalso be selectively controlled by varying the concentration and type ofemulsifier used during the polymerization reaction. In the presentinvention, the sodium lauryl sulfate can be changed with anionic,cationic and nonionic emulsifiers or the combinations of each. Specificexamples of suitable emulsifiers are sodium dodecyl sulfate, Dowfaxsurfactants, lgepal surfactants, Aerosol surfactants, Pluronicsurfactants, Cantrez surfactants and the like.

In view of the above rethinium tetroxide can be used as a staining agentin place and stead of the osmium tetroxide. Ruthenium tetroxide canstain a wide range of polymeric materials with varying structures andthe density of ruthenium tetroxide is lower than osmium tetroxide.Consequently, electrophoretic particles having a high degree ofblackness and a relatively low density can be created.

PROCESS THREE

In a third alterative embodiment of the present invention, blackparticles are produced by emulsion polymerization which producespoly(butadiene-co-styrene-co-methacrylic acid) latex particles. In thisembodiment poly(butadiene-co-styrene-co-methacrylic acid) particles areprepared utilizing the polymerization recipe shown below in Table IV.

                  TABLE IV                                                        ______________________________________                                        Materials         Weight (g)                                                  ______________________________________                                        styrene           12.0                                                        butadiene         30.0                                                        methacrylic acid  1.0                                                         distilled-deionized water                                                                       100.0                                                       Triton X-200      0.07                                                        Triton X-45       0.24                                                        potassium Persulfate                                                                            0.4                                                         divinylbenzene    0.4                                                         ______________________________________                                    

The inhibitors of the butadiene, styrene, and divinylbenzene are removedin the manner previously described. To produce the present inventionblack particles the Triton X-200 and Triton X-45 emulsifiers aredissolved in the distilled-deionized water and charged to a container.In accordance with Table IV, the styrene, potassium persulfate anddivinylbenzene are mixed and added to the emulsifier solution. Thecontainer holding the mixture is then purged with nitrogen and condensedbutadiene is added. The resulting mixture is then held at 70° C. andagitated for a desired reaction time of preferably forty eight hours. Asa result of the process, poly(butadiene-co-styrene-co-methacrylic acid)latex particles are formed.

The poly(butadiene-co-styrene-co-methacrylic acid) latex particles arestained black by being reacted with 2% by weight aqueous solution ofosmium tetroxide, ruthenium tetroxide or similar metal oxide. The metaloxide solution and poly(butadiene-co-styrene-co-methacrylic acid) latexparticles are allowed to react at room temperature for approximatelytwenty four hours, thereby staining thepoly(butadiene-co-styrene-co-methacrylic acid) latex particles black.The black particles are then collected for use in the electrophoreticdisplays.

As with previous embodiments, the size of black particles can becontrolled by varying certain process parameters such as reaction time,emulsifier concentration, temperature, etc. However, in the presentinvention particles size as well as the blackness and hardness of theend product products can be controlled by varying thebutadiene:styrene:methacrylic acid monomer ratio in the polymerizationrecipe. By varying the butadiene:styrene:methacrylic acid monomer ratiofrom 30:12:1, as shown in Table IV, to 30:11:2, 30:10:3, 20:22:1,20:21:2 and 20:20:3 the diameter of the resultingpoly(butadiene-co-styrene-co-methacrylic acid) latex particles can bevaried between 330 nm to 529 nm as desired. For example, apoly(butadiene-co-styrene-co-methacrylic acid) latex particle, producedas described above with a butadiene:styrene:methacrylic acid monomerratio of 20:20:3 produces latex particles having a diameter ofapproximately 390 nm as measured from a TEM photograph. Similarly,poly(butadiene-co-styrene-co-methacrylic acid) latex particles havingbutadiene:styrene:methacrylic acid ratios of 20:21:2 and 20:21:1 produceparticles having diameters of 480 nm and 510 nm, respectively, asmeasured from a TEM photograph.

The size of the poly(butadiene-co-styrene-co-methacrylic acid) latexparticle can also be selectively controlled by varying the concentrationand types of emulsifiers used during the polymerization reaction. In thepresent invention, particles can be controlled by varying the mount ofTriton X-200 emulsifier present from between 0.0 gms and 0.14 gms.Additionally, the amount of Triton X-45 emulsifier can be varied from0.16 gms to 0.40 gms to vary particle size. Anionic, cationic andnonionic emulsifiers or the combination of each type of emulsifier canbe used in the emulsion polymerization. Specific examples of suitableemulsifiers are sodium lauryl sulfate, sodium dodecyl sulfate, Dowfaxsurfacants, Igepal surfactants, Aerosol surfactants, Pluronicsurfactants, Cantrez surfactants, Arlacel sufactants, Tetronicsurfactants, poly(vinyl alcohol), poly(ethylene oxide), polyacrylic acidand the like.

The poly(butadiene-co-styrene-co-methacrylic acid) latex particle,although not technically a core/shell structure, is still dependent uponthe concentration of residual double bonds to determine how black theparticle will stain when reacted with a metal oxide. As with previousembodiments, the concentration of residual double bonds is dependentupon the degree of conversion, the ratio of monomers and concentrationof divinylbenzene in the polymerization recipe. The mount ofdivinylbenzene in the polymerization recipe of Table IV can be variedbetween 0.0 gms and 1.2 gms to create apoly(butadiene-co-styrene-co-methacrylic acid) latex particle with adesired residual double bond concentration and therefore a desiredblackness and hardness. In addition to butadiene monomer, otherconjugated diene compounds (e.g. isoprene) or any compound containingmore than one double bond (e.g. diacrylate, triacrylate, tetraacrylate,dimethacrylate and trimethacrylate) can also be used as a stainingcomponent. The degree of conversion can be varied from 50% to 99%depending upon the reaction time. A higher degree of conversion leads tofewer residual double bonds left in the final particles. Differentmonomer ratio give the final particles with varying blackness. A higherpercentage of butadiene monomer leads to more residual double bonds leftin the final particles.

The surface functionality of thepoly(butadiene-co-styrene-co-methacrylic acid) latex particle may beeffected by varying the amount of methacrylic acid monomer present inthe original polymerization recipe. Additionally, surface functionalitymay be controlled as desired by substituting methyl methacrylate,acrylonitrile, acrylic acid, vinyl acetate, sodium styrene sulfonate,cholorstyrene, Dimethylamino aminapropyl propylmethacrylamide,Isocyanatoethyl methacrylate, N-(iso-butoxymethyl) acrylamide, vinylchloride or other monomers in place and stead of the methacrylic acid.

PROCESS FOUR

In a fourth embodiment, black electrophoretic particles are made havinga polystyrene core structure and a poly(butadiene-co-methacrylic acid)shell structure. The polystyrene core structure, namely, seed particles,is prepared by emulsion polymerization pursuant to the polymerizationrecipe set forth below in Table V.

                  TABLE V                                                         ______________________________________                                        Materials         Weight (g)                                                  ______________________________________                                        styrene           40                                                          distilled-deionized Water                                                                       100                                                         potassium persulfate                                                                            0.4                                                         sodium lauryl sulfate                                                                           1.2                                                         divinylbenzene    0.4                                                         ______________________________________                                    

To produce the polystyrene seed latex, the inhibitors of the styrene anddivinylbenzene are removed and the styrene, water, potassium persulfate,sodium lauryl sulfate and divinylbenzene are charged into a container,purged with nitrogen and agitated at 60° C. for a desired reaction time.

Utilizing a seeded emulsion polymerization method thepoly(butadiene-co-methacrylic acid) shell structure is formed around thepolystyrene core structure. The poly(butadiene-co-methacrylic acid)shell structure is formed utilizing the polymerization recipe set forthin Table VI below.

                  TABLE VI                                                        ______________________________________                                        Materials         Weight (g)                                                  ______________________________________                                        polystyrene seed latex                                                                          100                                                         butadiene         20                                                          methacrylic acid  2                                                           distilled-deionized Water                                                                       80                                                          potassium persulfate                                                                            0.4                                                         divinylbenzene    4                                                           ______________________________________                                    

The poly(butadiene-co-methacrylic acid) shell structure is formed byfirst placing the polystyrene seed latex in a container with thebutadiene, methacrylic add and divinylbenzene, whereby the polystyreneseed particles are allowed to swell to a desired degree. The potassiumpersulfate initiator is then added to the container and thepolymerization reaction is held at 60° C. for a desired period ofpreferably forty-eight hours, thereby producing the desiredpolystyrene/poly(butadiene-co-methacrylic add) core/shell structure.

The resulting core/shell structure is then mixed with a 2% by weightaqueous solution of osmium tetroxide, ruthenium tetroxide or anothersimilar metal oxide, to produce a staining reaction with the residualdouble bonds of the core/shell structure. The staining thereby producingparticles of a desired blackness for use in the electrophoretic display.

In the present embodiment the core structure is derived from styrene. Itshould be understood that core/shell structures having differingdensities reflectivity, glass transition temperature and mechanicalstrength can be obtained by substituting either vinyl methyl ether,vinyl formal, vinyl alcohol, vinyl methyl ketone, acrylate,methacrylate, methyl, methacrylate, ethylene, propylene, like monomersor combinations thereof in place and stead of styrene.

Similarly, core/shell structures having different functional groups forsurface charging can be obtained by replacing the methacrylic acid inthe second stage polymerization recipe of Table VI with vinyl methylether, vinyl formal, vinyl alcohol, vinyl methyl ketone, acrylate,methacrylate, methyl methacrylate, dimethylaminopropylmethacrylamide,Isocyanatoethyl methacrylate, N-(iso-butoxymethyl) acrylamide, likefunctional monomers or combinations thereof.

Changes in residual bond density, particle size, blackness, hardness,and surface functionality can be affected in the manners previouslydescribed in relation to prior process embodiments.

PROCESS FIVE

In a fifth alternate embodiment, a multilayer composite structure isformed using a three step emulsion polymerization technique.

To create the original core structure a polystyrene core structure isformed in the manner previously described in connection with ProcessFour, following the first stage polymerization recipe set forth in TableV. The polystyrene core structure is then subjected to a secondarypolymerization process forming a first shell structure around thepolystyrene core. The first shell structure is prepared, as previouslydescribed in Process Four, utilizing the second stage polymerizationrecipe of Table VI, except the methacrylic acid deleted from thepolymerization recipe. As a result of the secondary polymerizationprocess, a polybutadiene shell structure is formed, thereby producing apolystyrene/polybutadiene core/shell latex.

The polystyrene/polybutadiene core/shell structure is then used as theseed for a third stage polymerization, wherein the polymerization recipefor the third stage polymerization is given in Table VII below.

                  TABLE VII                                                       ______________________________________                                        Materials         Weight (g)                                                  ______________________________________                                        polystyrene/      10                                                          polybutadiene seed latex                                                      styrene           10                                                          methacrylic acid  2                                                           distilled-deionized water                                                                       40                                                          potassium persulfate                                                                            0.4                                                         divinylbenzene    1                                                           ______________________________________                                    

To form the desired particle, the polystyrene/polybutadiene seed latexesare combined with the styrene, methacrylic add and divinylbenzene,wherein the polystyrene/polybutadiene particles are allowed to swell fora given period to reach a desired degree. The potassium persulfateinitiator is then added to the swelled polystyrene/polybutadieneparticles and the combination is tumbled at 60° C. for a predeterminedreaction time. The result of the third stage polymerization is amultilayered structure having two varied shell structures, wherein thefirst shell structure is a polybutadiene polymer and the second shellstructure is a poly(styrene-co-methacrylic acid) copolymer.

The multilayered structures are then combined with 2% by weight aqueoussolution of osmium tetroxide, ruthenium tetroxide, or like metal oxidesto produce the desired black electrophoretic particles.

Particles formed with a multilayer structure, wherein middle layer ispolybutadiene for providing the residual double bond needed forstaining, can be produced to have a higher degree of blackness, a lowerdensity and better mechanical properties than single shell structures byadding choices to the latexes that can be used to form theelectrophoretic particles. In view of the above description, it shouldbe understood by a person skilled in the art that multilayered compositelatex can be formed with any multitude of layers, each created by itsown unique polymerization recipe, so long as residual double bonds areformed at some point in the structure.

The described techniques for changing the particle size, residual doublebond density, blackness, hardness, and surface characteristics inprevious processes can be applied to the current multilayer structure.Additionally, multilayer structures of varying density, reflectivity,glass transition temperature and mechanical strength may be producedwhen the styrene, used in the original polymerization recipe, for thefirst stage polymerization process that produces the original corestructures, is changed to vinyl methyl ether, vinyl formal, vinylalcohol, vinyl methyl ketone, acrylate, methacrylate, methylmethacrylate, ethylene, propylene, other like monomers or combinationsthereof. The density, reflectivity, glass transition temperature andmechanical strength of the multilayer structure can also be controlledby effecting the polymerization recipe for the third stagepolymerization process as shown in Table VII. For example, density,reflectivity, glass transition temperature, and mechanical strength ofthe final particles can be altered as desired by replacing the styrenein Table VII with vinyl methyl ether, vinyl formal, vinyl alcohol, vinylmethyl ketone, acrylate, methacrylate, methyl methacrylate, ethylene,propylene, other like monomers or combinations thereof.

Additionally, the outer shell of the multilayer structure can be formedto have different functional groups for surface charging by varying thethird stage polymerization recipe. More particularly, the methacrylicacid listed in Table VII can be changed to vinyl methyl ether, vinylformal, vinyl alcohol, vinyl methyl ketone, acrylate, methacrylate,methyl methacrylate, ethylene, propylene,dimethylaminopropylmethacrylamide, isocyaratoethyl methacrylate,N-(iso-butoxymethyl) acrylamide or other similar functional monomers orcombinations of the above.

It should be understood that the processes for forming the presentinvention particles, as specifically described in this specification aremerely exemplary and a person skilled in the art may make numerousvariations and modifications to the describe processes without departingfrom the spirit and scope of the invention. More particularly, it willbe recognized by a person skilled in the art that many of the compoundsfound in the various polymerization recipes of Tables I through VII havechemical equivalents that have not been specifically stated. All suchequivalents are intended to be included in the scope of the invention.Additionally, it will be recognized by a person skilled in the art thatthe method set forth in each described process for varying the size,hardness, blackness, density, residual double bond density, surfacefunctionality, reflectivity, glass transition temperature and mechanicalstrength of the present invention particles can be applied to each otherdescribed process and a person skilled in the art may vary many otherparameters such as the time and temperature to also vary thecharacteristics of the final product black particles. All suchvariations and modifications are intended to be included within thescope of the invention.

PROCESS SIX

In a sixth alternative embodiment of the present invention blackelectrophoretic particles are produced by creating core/shell structureswherein there exists a polystyrene core and a poly(methacrylic acid)shell. In this embodiment the polystyrene core is made pursuant to thepolymerization recipe set forth below in Table VIII.

                  TABLE VIII                                                      ______________________________________                                        Materials        Weight (g)                                                   ______________________________________                                        styrene          40                                                           distilled-deionized                                                                            100                                                          potassium persulfate                                                                           0.4                                                          sodium lauryl sulfate                                                                          1.2                                                          divinylbenzene   0.4                                                          ______________________________________                                    

The contents of Table VIII are charged into a container, purged withnitrogen and agitated for a desired reaction time and at a desiredtemperature. In the preferred embodiment, the contents are tumbled at60° C. for thirty six hours. As a result of the polymerization reaction,polystyrene seed latexes are formed.

A second stage polymerization is then conducted utilizing thepolystyrene particles as the seed latexes. In the second stagepolymerization a poly(styrene-co-methacrylic acid) shell structure isformed around the polystyrene seed latexes. Thepoly(styrene-co-methacrylic acid) shell structure is formed utilizingthe polymerization recipe listed below in Table IX.

                  TABLE IX                                                        ______________________________________                                        Materials         Weight (g)                                                  ______________________________________                                        polystyrene       100                                                         seed latex                                                                    styrene           10                                                          methacrylic acid  2                                                           distilled-deionized water                                                                       40                                                          potassium persulfate                                                                            0.4                                                         divinylbenzene    0.4                                                         ______________________________________                                    

The polystyrene seed particles are swelled at room with the above listedmonomers for a predetermined reaction time. The potassium persulfateinitiator is then added to the container and the mixture is agitated fora desired reaction time. The resulting polystyrene/poly(styrene-co-methacrylic add) latex are then stained byadding a two percent by weight aqueous solution of ruthenium tetroxideto the containers, reacting the same until a desired degree of blacknessis obtained in the latex particles. Alternatively, the polystyrene/poly(styrene-co-methacrylic acid) latex may be exposed toruthenium tetroxide vapor to stain the polystyrene component.

The stained component in the present embodiment is the poly styrene. Itwill be recognized by a person skilled in the art that the stainedcomponent of styrene can be changed to vinyl methyl ether, vinyl formal,vinyl alcohol, vinyl methyl ketone, ethylene, propylene, other vinylmonomers or combinations of the above to make polymer particles havingdifferent properties such as density, reflectivity glass transitiontemperature and mechanical strength.

PROCESS SEVEN

In a seventh embodiment, black, electrophoretic particles are madehaving a hollow core structure and a multilayer shell structure.Firstly, a linear polystyrene core structure, namely, seed particle, isprepared by emulsion polymerization pursuant to the polymerizationrecipe set forth in Table X.

                  TABLE X                                                         ______________________________________                                        Materials         Weight (g)                                                  ______________________________________                                        styrene           40                                                          distilled-deionized water                                                                       100                                                         potassium persulfate                                                                            0.4                                                         sodium lauryl sulfate                                                                           1.2                                                         ______________________________________                                    

The polystyrene seed particles are then subjected to a secondaryemulsion polymerization forming a first shell structure around thepolystyrene core. The first shell structure is prepared, as previousdescribed in Process Four, utilizing the second stage polymerizationrecipe of Table VI. As a result of the secondary polymerization process,a polybutadiene shell structure is formed, thereby producing apolystyrene/polybutadiene core/shell structure.

The polystyrene/polybutadiene core/shell latex used as the seed is thensubjected to a third stage polymerization process forming a second shellstructure around the polystyrene/polybutadiene core. The second shellstructure is prepared, as previously described in Process Five,utilizing the third stage polymerization recipe of Table VII. The resultof the third stage polymerization is a multilayered structure having twovaried shell structures, wherein the first shell structure is apolybutadiene polymer and the second shell structure is apoly(styrene-co-methacrylic acid) copolymer.

Because the polystyrene core is a linear polymer which is soluble in itsgood solvents, such as tetrahydrofurane and toluene, the finalmultilayered composite particles (after the third stage polymerization)are mixed with a large amount of good solvent (e.g. toluene) and tumbledat room temperature for 48 hours to dissolve the linear polystyrene outof the core of the composite particles. The dissolved polystyrene andthe good solvent are then removed resulting a multilayered compositestructure with a hollow core.

The multilayered hollow composite particles are then combined with 2% byweight aqueous solution of a metal oxide to produce the desired blackelectrophoretic particle.

Particles formed with a hollow structure can be produced to have adensity close to or lower than 1 g/cm³ which can be easily dispersed ina low-density medium (e.g. decane and octane) without adding anotherhigh-density medium (e.g. carbon tetrachloride and tetrachloroethylene)and are easier to remain a good colloidal stability.

Particles formed with a multilayer structure, wherein middle layer ispolybutadiene for providing the residual double bond needed forstaining, can be produced to have a high degree of blackness, a lowdensity and good mechanical properties by adding choices to the latexesthat can be used form the electrophoretic particles. In view of theabove description, it should be understood by a person skilled in theart that multilayered hollow composite latex can be formed with anymultitude of layers, each created by its own unique polymerizationrecipe, so long as residual double bonds are formed at some point in thestructure.

The described techniques for changing the particle size, residual doublebond density, blackness, and surface characteristics in previousprocesses can be applied to the current hollow structure. The density,reflectivity, glass transition temperature and mechanical strength ofthe multilayer hollow structure can also be controlled by effecting thepolymerization recipe for the third stage polymerization process asshown in Table VII. For example, density, reflectivity, glass transitiontemperature, and mechanical strength of the final particles can bealtered by replacing the styrene in Table VII with vinyl methyl ether,vinyl formal, vinyl alcohol, vinyl acetate, vinyl methyl ketone,acrylate, methacrylate, methyl methacrylate, ethylene, propylene, otherlike monomers or combinations thereof.

Additionally the outer shell of the multilayer structure can be formedto have different functional groups for surface charging by varying thethird stage polymerization recipe. More particularly, the methacrylicacid listed in Table VII can be changed to vinyl methyl ether, vinylformal, vinyl chloride, vinyl methyl ketone, vinyl acetate, acrylic add,sodium styrene sulfonate, methyl methacrylate,dimethylaminopropylmethacrylamide, isocyanatoethyl methacrylate,N-(iso-butoxymethacrylamide), other like functional monomers orcombinations of the above.

Furthermore, in the described processes of the present invention batchemulsion polymerization was used. In addition to the latexes orcore/shell composite latexes made by the emulsion polymerization orseeded emulsion polymerization, the polymer particles containing ether,arene, alcohol, aromatic, amide or olefin moieties or any combination ofthe above moieties, made by emulsion polymerization, miniemulsionpolymerization, microemulsion polymerization, suspension polymerization,dispersion polymerization or precipitation polymerization can also beused for staining with ruthenium tetroxide to make non-conductive blackpolymer particles with controlled properties.

All equivalents, variations and modifications that can be applied to thedescribed present invention by a person skilled in that art, areintended to be included within the scope of this invention as defined bythe appended claims.

What is claimed is:
 1. Multilayer dielectric particles comprising acomposite latex polymer having at least one core structure and a shellstructure, said core structure having residual double bonds which havereacted with a metal oxide that produces a black color.
 2. The particlesaccording to claim 1, wherein each of said particles is a compositelatex having a core structure and at least one shell layer formed by aseeded emulsion polymerization technique.
 3. The particles according toclaim 1, having a plurality of shell layers including an outermost shelllayer and a second outermost shell layer closest to the core and whereinsecond outermost shell layer contains said residual double bonds.
 4. Theparticles according to claim 2, wherein said core structure comprises apoly(butadiene-co-styrene) polymer.
 5. The particles according to claim4, wherein said at least one shell layer comprises apoly(styrene-co-methacrylic add) polymer.
 6. The particles according toclaim 2, wherein said core structure comprises a polystyrene polymer. 7.The particles according to claim 6, wherein said at least one shelllayer comprises a polybutadiene polymer.
 8. The particles according toclaim 6, wherein said at least one shell layer comprises apoly(styrene-co-methacrylic acid) polymer.
 9. The particles according toclaim 8, wherein said polystyrene polymer core structure is surroundedby at least one shell layer wherein polybutadiene polymer said at leastone shell layer polybutadiene polymer is surrounded by at least oneshell layer poly(styrene-co-methacrylic acid) polymer.
 10. The particlesaccording to claim 1, wherein said metal oxide comprises osmiumtetroxide.
 11. The particles according to claim 1, wherein said particleis hollow, having a hollow core structure encapsulated by at least oneshell layer.
 12. The particles according to claim 1, wherein saidcomposite latex polymer is a poly(butadiene-co-styrene-co-methacrylicacid) polymer.
 13. A multilayer dielectric particle comprising a latexshell structure anda latex core structure having residual double bondswhich have reacted with and been stained black by a metal oxide.
 14. Amultilayer dielectric particle comprisinga composite latex polymerhaving a core structure and at least one outer shell structure, whereinsaid core structure has residual double bonds that have reacted with ametal oxide which produces a black color, wherein said metal oxide isselected from the group consisting of osmium tetroxide and rutheniumtetroxide.